What causes stiffness and pain in our muscles for days after exercise

Normally, when our muscles contract they shorten and bulge, much like a bodybuilder’s biceps. But if the muscle happens to be stretched as it contracts it can cause microscopic damage. The quadriceps muscle group located on the front of the thigh is involved in extending the knee joint, and usually contracts and shortens to straighten the leg. However, when walking down a steep slope, the quadriceps contract to support your body weight as you step forward, but as the knee bends, the muscles are pulled in the opposite direction. This tension results in tiny tears in the muscle and this is the reason that downhill running causes so much delayed-onset pain. A muscle is made up of billions of stacked sarcomeres, containing molecular ratchets that pull against one another to generate mechanical force. If the muscle is taut as it tries to contract, the sarcomeres get pulled out of line, causing microscopic damage. The muscle gets inflamed and fills with fluid, causing stiffness and pain.

 

The idea behind resistance training is that you're basically tearing something and creating a micro trauma in the muscle. When the muscle recovers, it's going to recover stronger and denser than it was before. The soreness you feel the day after an upper-body workout—when you're hauling groceries into your car and you can hardly lift your arms is good.

Just make sure what you're suffering from is DOMS and not an injury. A good way to tell the difference is if the pain is bilateral. Having one very sore shoulder after you've worked both shoulders could spell injury.